Field of the Invention
The invention relates to a method and an apparatus for detecting and correcting a spatial position of a workpiece held in a positioning device.
Description of the Background Art
Various methods for correcting the position of workpieces to be machined are known. For example, a method for setting the position of a tool by a drive system is disclosed in DE 101 39 081 A1. In order to achieve a correction of deviations between a momentary position value and an actual position value of a tool that do not actually coincide with the aimed-for target position value on account of required computing times of a control device and other events that have a delaying effect, the setting of the position of the tool is performed exclusively by the evaluation of data that are provided by a sensor device.
DE 10 2009 026 484 A1 relates to an apparatus and a method for machining workpieces. The object of DE 10 2009 026 484 A1 is based on the problem that relatively large apparatuses (machining apparatuses), such as turning or milling machines, in particular first have to reach a certain operating temperature in order for the infeeding paths and relative positions that are provided in the structural design to be precisely correct. For this purpose, it is envisaged to sense a difference occurring between desired and actual positions by means of a sensor. The sensor may in this case be arranged on a holding device or on a machining head. The actual distance is sensed between the holding device and the machining head and is compared with a desired distance. In dependence on the amount of a difference formed between the desired distance and the actual distance, control of the machining of the workpiece is subsequently possible.
The disclosure of DE 10 2011 006 447 A1 then relates to a method for machining structurally identical workpieces by means of an apparatus in which, for the machining, tools are guided along actual tool paths. These actual tool paths are based on known desired tool paths, which in turn represent expected tool paths. During a performed machining of the workpiece, distance values between the desired tool path and the workpiece are continuously measured by means of a sensor. On the basis of these distance values, an actual tool path of the tool is determined and its path deviation from the desired tool path is ascertained. The path deviation is determined and it is checked whether it lies within admissible tolerance values. The desired tool path is corrected on the basis of the distance values and the corrected desired tool path is possibly used as a basis for controlling the tool for subsequently machined workpieces. In principle, in a method according to DE 10 2011 006 447 A1 it is assumed that the workpiece is located at a correct position and the workpiece has at most production-related tolerances. An actual spatial position of the workpiece is not determined.
To be mentioned as the closest prior art is the applicant's as yet unpublished application DE 10 2012 109 245, in which a method and an apparatus for machining non-rotationally symmetrical workpieces by means of laser radiation are described. The disclosed apparatus comprises an infeeding device for infeeding the workpieces to a first machining position and with a machining device having at least one machining head for the machining of a workpiece infed to the first machining position. The infeeding device is a gripping robot with a gripping device. Arranged in the range of its grasp is a providing device with holders, the design and dimensioning of which is chosen such that at least one workpiece held in the holders is provided, with a defined spatial alignment and position of the infeeding device for being removed from the holders. At least one clamping device is also arranged, for receiving a workpiece infed by the infeeding device, a machining position being respectively defined by each clamping device. The clamping device and the infeeding device may also be regarded as a positioning device, since the workpiece is positioned by the interaction of the two. The machining head can be infed to each machining position for the machining of the workpiece.
The solution disclosed in the aforementioned DE 10 2012 109 245 ensures that the workpieces are theoretically infed (positioned in relation) to the machining apparatus in an exact spatial position. The machining apparatus starts the machining with pre-programmed contours, which the laser beam machines on the workpiece.
A pre-requisite for this solution from DE 10 2012 109 245 to work is that workpieces are within a prescribed tolerance range. In particular for the machining of curved workpieces, it is essential that they do not exceed the predefined tolerances in their geometrical dimensions. The workpieces and the machining head are infed to the machining positions with a tolerance of less than 0.1 mm.
It cannot be ensured in industrial practice that the tolerance ranges of the workpieces can be kept so small that the very small tolerance values are maintained during the infeeding of the workpieces and the machining head to a machining position. Sources of error for dimensional deviations are especially the radii of curvature of the workpieces. Such workpieces are often produced by bending a semifinished product, for example a pipe. The material stresses thereby produced in the workpiece may, for example when acting together with effects of the ambient temperatures on the material, lead to an unpredictable deformation of the workpiece. Even small deviations, for example in the radii of curvature, lead to local displacements in the position of the workpiece that considerably impair or completely prevent the machining result. A further problem is the risk of a collision between the machining head and the workpiece, whereby the machining apparatus may be maladjusted or even destroyed.
However, an elaborate measurement of each workpiece would slow down the production sequence considerably. Therefore, a solution that prevents a collision, improves the quality of the machining and can be integrated in the machining process is sought. It is also intended for it to be checked retrospectively whether the machining process has been successfully completed.